1. Field of the Invention
The present invention relates to mobile communication systems. In particular, the present invention relates to a method and an apparatus for searching multipaths of a mobile communication system at a higher speed by applying different weights to different pilot symbol numbers, using a dedicated physical control channel (hereinafter, it is referred to as DPCCH) in which data is received in a reverse link of asynchronous international mobile telecommunication for the 2000s (IMT-2000).
More particularly, the present invention relates to a method and apparatus for searching multipaths of a mobile communication system at a higher speed by multiplying different channel weights on different DPCCH energies in accordance with a spreading factor of the reverse dedicated physical data channel (hereinafter, it is referred to as DPDCH), given that a base station is aware of the spreading factor.
In general, a reverse link dedicated physical channel (hereinafter, it is referred to as DPCH) in the mobile communication system is divided into the DPDCH for transmitting data and DPCCH for transmitting control information. The channels are multiplexed into I/Q code, respectively, in a separate wireless frame before they are received.
Since a transmission signal in the wireless communication system is sent out to a receiver via multipaths, it is susceptible to many scattering objects, such as buildings, trees, or other moving objects. In addition, having compared an electromagnetic wave that travels through a direct path between a transmitter and a receiver with a radio path influenced by a number of scattering objects, the inventors learned that the field intensity at a receiving site varies on an hourly basis due to the attenuated signals and time delay en route. Therefore, by way of overcoming such problems, a new method to acquire faster multipaths in the mobile communication system has been studied.
Specifically, according to an aspect of the present invention, in case of searching the multipaths using an up-link DPCH without knowing the spreading factor of the reverse DPDCH, a variable weight is multiplied to a pilot section of the DPCCH land the other control symbol sections in order to find a total energy.
Another embodiment of the present invention finds the total energy by using the DPCCH and DPDCH, given that the spreading factor of the DPDCH is known, and obtains the total energy by multiplying a corresponding channel weight. On the top of that, the DPCCH and DPDCH can be selectively used for helping the base station to find multipaths of a mobile station a lot faster.
2. Description of the Related Art
The standards of Code Division Multiple Access (CDMA) in the asynchronous IMT-2000 system suggest that the base station transmit a pilot channel or a pilot symbol not only in a forward link but also receive one in a reverse link. Therefore, a receiver of the CDMA communication system based on the new standard should be able to conduct the search using the energy of the pilot signal both in the forward link and the reverse link.
Because the mobile communication system is usually l-to-N communication, the pilot channel of the forward link is a common element of all mobile stations within a cell zone of the corresponding base station, and it takes a lot of power to transmit the pilot channel. In contrast, the dedicated pilot channel of the reverse link is dedicated to an individual mobile station so that it can be transmitted at a low power. Of course, the intensity of the power (i.e., energy) of the dedicated pilot channel in the reverse link can be varied a little, depending on the standards or data transmission rates.
In order to find reverse multipaths in the asynchronous IMT-2000 system, the reverse DPCH shown in FIG. 1 is generally used. The DPCH is constituted of a time slot having 15 radio frames with a fixed or constant frequency (Tf=10 ms), and it includes the DPDCH and the DPCCH.
Normally, a time slot is constituted of 2560 chips. Especially the 2650 chips of the DPCCH includes a pilot symbol section (Npilot bits), and a transport format combination indicator, TFCI (NTFCI bits) as a control symbol, and feedback information, FBI (NFBI bits), and transmit power control information, TPC (NTPC bits).
According to one of traditional CDMA methods of searching multipaths, the dedicated channel in the communication system was coherently accumulated on a symbol basis for calculating energy. And, the calculated energy value was then noncoherently accumulated to a certain number, which was eventually used for calculating the energy value of the multipaths.
FIG. 2 is a block diagram illustrating a configuration of a receiver for a CDMA communication system in the related art.
Referring to FIG. 2, a first multiplier 101 in the receiver for the traditional CDMA communication system separates an inputted signal through an antenna into I channel and Q channel, respectively, and the separated signal is filtered out at a Filter 102. Then a second multiplier 104 multiplies the filtered signal by a pseudo noise (PN) code generated by a pseudo noise code generator 108 of a searcher 103, and outputs the result. On the other hand, a complex output signal is synchronized and accumulated in a coherent accumulator 105. And, an energy detector 106 detects energy, and the detected energy is later accumulated in a noncoherent accumulator 107. Here, the accumulation procedure and the energy detection procedure are carried out in a branch to measure the intensity of power (energy) of the pilot channel.
In addition, a threshold comparer 110 in a controller 109 compares the accumulated signal in the noncoherent accumulator 107 of the searcher 103 with a threshold, and assigns a finger to an optimum signal path. Each finger 111 & 112 assigned to the optimum signal path by the controller 109 performs a demodulation process.
In short, the receiver in the traditional CDMA communication system searches multipaths using a branch only to measure the intensity of power of the pilot channel. However, it is known that the branch is good only for the forward link having high power for the pilot channel, while the overall performance of the branch is deteriorated in the reverse link having low power for the pilot channel.
Besides, if the data rate of a DPDCH signal is high, the signal intensity of the DPDCH is high. In contrast, if the data rate of the DPDCH signal is relatively low, the signal intensity of the DPDCH gets weak. That is, the signal intensity is proportional to the data rate. Meanwhile, the receiver can find out the data rate only when the TFCI is duly received to 1 frame, which also means that if the data rate is not properly received to the frame, there is no way for the receiver to know the data rate.
Unlike the traditional systems like AMPS or CDMA, the IMT-2000, known as the third generation mobile communication, introduces a new method for searching multipaths, which is not even founded in the current CDMA standards.